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16th-century view of Itzehoe
Itzehoe is one of the oldest towns in Holstein.
As early as 810 AD, Charlemagne built the Esesfeld castle in the Oldenburgskuhle, 2 kilometres from the later town, as protection against the Danes marauding from the north, but this has no direct connection with the development of Itzehoe. Under its protection, Archbishop Ebbo of Reims built a small monastery or prayer house, the ‘cella Welanao’, in the summer of 823 in what is now Münsterdorf as a base for the Christian mission he initiated in Denmark. The larger Echeho Castle, built around 1000 in the nearby meander of the River Stör, became the nucleus of a settlement that developed into a trading town, favoured by the granting of the Lübeck rights (1238), combined with freedom from customs duties, which at that time was only granted to Hamburg in the country, and later the right to stack goods (1260). During this time, Itzehoe was involved in the salt, cloth and grain trade and was at times an important hub in European east–west trade. Further settlements developed on the other side of the river around the monastery courtyard (around 1260) and around the Church of St. Laurentii (first mentioned in 1196).
As part of the Duchy of Holstein, Itzehoe remained under the rule of the Danish crown from 1460 until 1864.
In the 16th century, the proximity to Burg Breitenburg, the ancestral seat of the Rantzau family, who were the governors of the Duchies of Schleswig and Holstein on behalf of the Danish king, was particularly beneficial. Heinrich Rantzau, in particular, expanded the economic infrastructure of the surrounding area and promoted the Itzehoe monastery. He also built a city palace on Breiten Straße, which no longer exists today, and expanded the Rantzau family's burial grounds in the church. He also initiated the installation of the first organ in St. Laurentii Church and the promotion of church music. The first depiction and written description of the city in Braun and Hogenberg's map collection was commissioned directly by Heinrich Rantzau.
During the Thirty Years' War, the town was repeatedly occupied and plundered, but there was no major destruction because the town council surrendered the town to General Wallenstein without a fight in 1627. This allowed Itzehoe to maintain its status as the fifth of the eighteen towns of Schleswig-Holstein.
After having been largely spared from war for a long time, Itzehoe was almost completely destroyed by Swedish soldiers in 1657 during the Danish-Swedish War (1657–1658). As a result, the cloister of St. Laurentii Church is now the only surviving medieval building in Itzehoe.
In 1712, the Asian bubonic plague, which had been brought in from East Prussia and Poland, broke out in Itzehoe. The disease claimed the lives of 250 inhabitants (around 7% of the population of 3,500 at that time).
Itzehoe was only indirectly affected by the Napoleonic Wars through transit and quartering as well as financial burdens. From 1807, Itzehoe briefly became the residence of Elector Wilhelm I of Hesse-Kassel, who had fled into exile from Napoleon.
From the first two decades of the 19th century onwards, a lively cultural life developed in Itzehoe. This was further promoted by the presence of German-Danish nobility in the administration and military. From the 1810s onwards, the first ‘reading societies’ and choirs were founded. At the same time, several printing houses were established and, in 1817, the Itzehoer Wochenblatt, the town's first newspaper, was launched, which soon became important beyond the region.
Before the Schleswig-Holstein uprising, in which a large part of the citizens of Itzehoe took sides with the pro-German Schleswig-Holstein movement, the Holstein Assembly of Estates met in Itzehoe from 1835 to 1848 and again from 1852 to 1863, thus establishing the history of parliamentarianism in Schleswig-Holstein.
After the German-Danish War of 1864, the Duchy of Holstein was initially administered jointly by Prussia and Austria, ending over 400 years of Danish rule. The new governor Ludwig Karl Wilhelm von Gablenz finally convened the Holstein Assembly of Estates for the last time on 11 June 1866. However, a conference was prevented by the events surrounding the Austro-Prussian War of 1866. After the end of this war, the Duchy of Holstein, including Itzehoe, fell to Prussia: in 1867, together with the Duchy of Schleswig, the province of Schleswig-Holstein was created.
The railway connection (1847) and the link to the new road from Hamburg to Rendsburg (1846) ushered in the industrial age in Itzehoe, with many commercial and industrial enterprises (including sugar production, weaving, chemical industry and shipbuilding) settled in and around Itzehoe, helping the town regain its economic importance.
Itzehoe was listed as a garrison depot (Wehrkreis X (Hamburg)) of the former 225th Infantry Division, which was implicated in the 1940 Vinkt Massacre in Belgium.
Following the joint German-Soviet invasion of Poland, which started World War II in September 1939, it was the location of the Oflag X-A prisoner-of-war camp for Polish officers, which was eventually relocated to Sandbostel in 1941. During the war, Itzehoe was not initially a primary target for the Allied strategic bombing campaign, but was hit by an Allied bombing raid very late in the war, when Allied carpet bombing on 2 May 1945 (just two days before the German surrender at Lüneburg Heath) caused 22 fatalities in the city.: 436 
Until it was filled in with around 110,000 m3 of sand in 1974, a bend in the River Stör had a decisive influence on the appearance of Itzehoe town centre. The bend was the original course of the river. The Stör crossing ( low German ‘Delf’, from which the names “Delftor” and “Delftorbrücke” of the town exit and the Störbrücke bridge originate) turned Itzehoe's castle complex into an island. There are said to have been sluices in the delta that closed when the water ran out, forcing the water to flow through and clean the bend. After their removal, the river increasingly silted up and developed into an almost stagnant, foul-smelling body of water. The old town centre, the ‘Neustadt’ (new town), could only be reached via bridges. In the course of the redevelopment of the ‘Neustadt’, during which almost all the houses on this former island were demolished and replaced by new buildings and new streets were laid out, this element that characterised the town became extinct. Only a few artificially created water basins between the new theatre and Salzstraße are reminders of the original course of the loop.
In order to improve the cityscape again, an initiative was launched in 2011 with the aim of promoting the reopening of the filled-in Störschleife in the centre of Itzehoe. In 2017, the entire city centre was declared a redevelopment area. The restoration of the Störschleife was explicitly named as a goal.
On 26 September 2021, the residents of Itzehoe voted by 7707 votes in favour of restoring the Stör bend. Concrete plans for implementation have been underway since August 2022. In February 2023, the winning design was to be presented by one of nine planning teams.
In the 1990s, the Fraunhofer Institute for Silicon Technology ISIT was established on the northern edge of Itzehoe. Many chip research and high-tech companies have since settled around this institute on the InnoQuarter Itzehoe industrial estate. As a result, Itzehoe is increasingly developing into a centre of high technology in northern Germany.
